Slice the flank steak into thin, bite-sized strips and place them in a small mixing bowl.
Toss the steak strips with the chili powder, cumin, sea salt, black pepper, and the juice of the half lime until evenly coated.
Heat the olive oil in a large non-stick skillet over medium-high heat until shimmering.
Add the seasoned steak to the skillet and sear for 3-4 minutes until browned and cooked through, then remove the steak and set aside.
In the same skillet, sauté the sliced bell peppers and red onions for 3-5 minutes until they are tender and slightly charred.
Wipe the skillet clean and return it to medium heat, then place the whole wheat tortilla in the pan.
Sprinkle half of the shredded cheddar cheese onto one half of the tortilla, then layer on the cooked steak and sautéed vegetables.
Top the filling with the remaining cheese and the fresh cilantro, then fold the tortilla over to close.
Cook the quesadilla for 2-3 minutes per side, pressing down with a spatula, until the tortilla is golden brown and the cheese is fully melted.
Remove from the pan and let rest for one minute before slicing into wedges.